Electroless coating of molybdenum on stainless steels

ABSTRACT

AN ADHERENT COATING OF MOLYBDENUM ON STAINLESS STEEL IS OBTAINED BY CONTACTING THE STAINLESS STEEL WITH A MOLTEN FLUORIDE SALT CONTAINING FROM 1 TO 5 WEIGHT PERCENT MOF6.

United States Patent Office 3,783,014 Patented Jan. 1, 1974 US. Cl.117-130 R 2 Claims ABSTRACT OF THE DISCLOSURE An adherent coating ofmolybdenum on stainless steel is obtained by contacting the stainlesssteel with a molten fluoride salt containing from 1 to 5 weight percentMoF BACKGROUND OF THE INVENTION The invention described herein was madein the course of, or under, a contract with the US. Atomic EnergyCommission.

The present invention relates to a method for depositing a coherent,adherent coating of molybdenum on a stainless steel substrate and to acomposition for practicing the method.

The prior art known to the applicants for coating stainless steels withmolybdenum consists of electrolytic plating and chemical vapordeposition processes. Both techniques are effective for depositingmolybdenum, but are of limited utility in applying a uniform coating toa surface of complex geometry such as the internal surfaces of a pipingarrangement or a vessel with relatively inaccessible surface area. Inelectrolytic processes it is difficult to attain uniform throwing power,and the composition of the electrolytic bath as well as electroplatingprocess parameters must be constantly monitored. Molybdenum cannot besatisfactorily deposited directly upon a stainless steel surface butrequires an initial application of a nickel coating to insure goodadhesion of molybdenum.

SUMMARY OF THE INVENTION The present invention is not subject to thelimitations of electroplating or chemical vapor deposition. Itcomprises, in its process aspect, formulating a molten fluoride saltsolvent phase containing from 1 to about 5 weight percent molybdenumfluoride, based on the weight of the solvent phase, and contacting theresultant molybdenum-containing molten salt composition with an iron-,nickel-, or chromium-containing substrate for a time suflicient toeffect deposition of molybdenum by a replacement chemical reaction. Themolten fluoride solvent phase may consist of an alkali metal fluoride,an alkaline earth fluoride, or any mixture thereof. Typical solventsalts which can be used to practice this invention are listed below:

Solvent salt: Liquidus C.) NaF-LiF-KF (11.5-46.5-42.0 mole percent) 459NaF-LiF-BeF (27-35-38 mole percent) 338 NaF-BeF -KF (30-65-5 molepercent) 435 NaF-KF-ZrE; (5-52-43 mole percent) 425 KF-NaF-ZrF;(18-36-46 mole percent) 450 LiF-BeF (69-31 mole percent) 505 ofdeposition may vary from as low as 300 C. to as high gaseous mixture ofMoF and an inert carrier gas into the volume of the fused fluoride saltphase until it is present in solution to at least about 1 weight percentMoF Lower amounts may be useful in selected circumstances, but for mostapplications a minimum of 1 percent molybdenum fluoride is preferred.

The exact amount of molybdenum required for a particular application canbe approximated from and is a function of the composition of thesubstrate material in the light of the following apparent mechanism ofdeposition. The deposition process is visualized as taking place by areplacement chemical reaction between molybdenum as the metalconstituents of the substrates according to the formula M=Fe, Cr, or Ni;a=substrate constituent; s=solution;

d=deposit; and n=valence of M.

This reaction involves the oxidation of Fe, Cr, and Ni to theirrespective fluorides by the reduction of the molybdenum fluoride. Theexperimental evidence in support of this mechanism is based on analysisof the fluoride salt phase after molybdenum deposition which showsconcentrations of Fe, Cr, and Ni in solution at levels comparable to theamount of molybdenum deposited on the metal surface.

To insure continuous adherent coating of molybdenum, the conditions mustbe controlled such that the substrate fluorides do not exceed theirsolubility in the molten fluoride composition. Otherwise, theundissolved metal fluorides will codeposit with molybdenum to produce anonuniform deposit. Solubility can be maintained by temperature and bycontrolling the amount of molybdenum in solution. At constanttemperature, the main controlling variable is the concentration ofmolybdenum. For most applications, the maximum concentration ofmolybdenum fluoride in the salt phase should not exceed 5 weight percentin order to avoid exceeding solubility limits of the substrate metalfluorides.

DESCRIPTION OF A REPRESENTATIVE EMBODIMENT A sample of Type 316stainless steel consisting essentially of 17 weight percent Cr, 12weight percent Ni, 2 weight percent Mo, and the balance Fe was coatedwith molybdenum according to the following procedure.

Three separate volumes of a molten fluoride salt consisting essentiallyof UP and BeF (about 2:1 mole percent) were heated to 675 C. A gaseousmixture of MoF in a helium carrier was then bubbled through each of themelts until concentrations of 1, 4, and 10 weight percent MoF wereestablished in the respective melts. Sample coupons of Type 316stainless steel were then immersed in each of the molybdenum-containingmelts for a period ranging from 50 to hours at melt temperatures rangingfrom 650 to 675 C. The coupons were removed and found to have beencoated with molybdenum. An adherent and uniform coating of molybdenumwas obtained from the melts containing 1 and 4 weight percent molybdenumfluoride. Microprobe analyses indicated that the molybdenum had diffusedinto the substrate to a depth of at least 350 microns. While the sampleimmersed in the bath containing 10 weight percent molybdenum had acontinuous coating, some areas were found to be easily removed.Metallographic and microprobe analysis of the samples showed amultilayered coating containing fluorides of lithium, beryllium, iron,and chromium. In this case, the solubility limits of the fluorides ofthese elements had been exceeded, causing co-deposition of thiscorrosion product with the molybdenum. Thus it is these fluorides whichappeared to be largely responsible for the non-adherence of themolybdenum.

It will be see that a general method has been described for depositing auniform adherent deposit of molybdenum on a metal substrate. The methodis simple and uncomplicated, and has unique capability in applying auniform and adherent molybdenum coating on remote surfaces and articlesof complicated geometry. A singularly apt application of this process isfound in the preparation of apparatus for the chemical processing ofspent molten fluoride nuclear fuel compositions involving selectiveextraction of metal values in the fluoride phase into a liquid bismuthphase. A molybdenum-surfaced container system is needed to withstand thecorrosive effects of both molten fluorides and liquid bismuth. With themolybdenum coating process of this invention at hand, it is now possibleto construct an entire container system together with associated pipingfrom stainless steel and then simply fill the system at the desiredtemperature with the molybdenum-containing fused fluoride composition toachieve a uniform adherent coating throughout the system. Circulation ofthe salt through the system will coat all vessels and interconnectedpiping in the system.

What is claimed is:

1. A method for electrolessly applying an adherent coating of molybdenumon a stainless steel substrate which comprises contacting said substratewith a fused salt comprising at least one alkali metal fluoride,alkaline earth metal fluoride, or mixtures thereof and from 1 to 5weight percent molybdenum, as MoF dissolved in said salt at atemperature and for a period of time sufficient to coat said substrate.

2. The method of claim 1 wherein the temperature of deposition is in therange 300 to 800 C.

References Cited UNITED STATES PATENTS 3,444,058 5/1969 Mellors et al.204-39 X 2,734,003 2/1956 Alpert 117-130 R 2,746,888 5/1956 Ross 1171l3X 2,786,809 3/1957 Raynes 204-39 2,828,251 3/1958 Sibert et al. 204392,872,348 2/1959 Eubank 117-130 R X 2,950,233 8/1960 Steinberg et a120439 X 3,203,811 8/1965 Boller 117130 R X ALFRED L. LEAVITT, PrimaryExaminer J. R. BATTEN, 1a., Assistant Examiner US. Cl. X.R.

